Sean Young -- Off the Hook After Guard-Slapping Oscars Arrest

Sean Young won't face any charges after allegedly slapping a security guard at an Oscars after party last week -- this according to the L.A. City Attorney.

You'll recall ... the "Ace Ventura: Pet Detective" star was arrested at the Governors Ball when she got into a dispute with a security guard, who accused her of not having a ticket. We're told the guard put his hand on Sean, and she slapped him.

Sean was arrested and booked on misdemeanor battery -- but a rep for the L.A. City Attorney's office tells TMZ, "Our prosecutors declined to file charges ... due to insufficient evidence."

The rep adds, "No further action is expected."